A number of useful substances are produced in and isolated from the culture broth of various strains of the genus Streptomyces. It is known that Streptomyces myxogenes SF-1130 (identified as FERM-P. 676) produces an antibiotic called SF-1130 substance (see Japanese Patent Publication No. 30393/73). We have made further research on the fermentation broth of the microorganism Streptomyces myxogenes, SF-1130, and we have now found that new antibiotic substances active against gram-negative bacteria are produced in the fermentation broth of said microorganism. We have succeeded in isolating these active substances from the broth and have designated them as SF-1130-x.sub.1 substance and SF-1130-x.sub.2 substance, respectively.
We have further found that these two active substances exhibit an activity by which the immunity inherently involved in living animals and man is prevented from being reduced more or less due to the formation of a tumor and/or due to the administration of an immunosuppressive anti-tumor agent, and that these active substances are hence useful as an immunopotentiator for enhancing the immune response in living animals and men.
An object of this invention is to provide new antibiotics which are useful as an antibacterial agents and as an activator for enhancing the immunity in living animals and men. A further object of this invention is to porvide a process of producing these useful substances. Other objects will be clear from the following descriptions.